Currently, a major trend in the display market is moving from existing high-efficiency and high-resolution oriented displays to the emotional image-quality displays having high color purity for demonstration of natural colors. From this viewpoint, currently, organic light-emitter based organic light emitting diode (OLED) elements have achieved rapid progress, and inorganic quantum dot light emitting diodes (LEDs) with improved color purity have been actively researched and developed as alternatives. However, both an organic light-emitter and an inorganic quantum dot light-emitter have inherent limitations in terms of materials.
The existing organic light-emitter has high efficiency, but has a wide spectrum and poor color purity. The inorganic colloidal quantum dot light-emitter has been known to have good color purity. However, there is a problem in that color purity is reduced because it is difficult to control a size of a quantum dot uniformly as the color approaches the blue color because of size distribution. Furthermore, because inorganic quantum dots have a very deep valence band, there is a problem in that hole injection barriers in an organic hole injection layer are too large to inject holes. Also, there is a disadvantage in that the two light-emitters are expensive. Therefore, there is a need for a new type of organic/inorganic/hybrid light-emitter that compensates for the disadvantages of the organic and inorganic light-emitters and maintains advantages thereof.
Since hybrid materials composed of organics and inorganics have both advantages of organic materials such as low manufacturing cost, simple producing and element manufacturing processes, and easily controllable optical and electrical properties and advantages of inorganic materials such as high charge mobility and mechanical and thermal stability, the organic/inorganic/hybrid materials have been spotlighted both academically and industrially.
Since organic/inorganic/hybrid perovskite materials, hereafter “perovskites”, among the organic/inorganic/hybrid materials have high color purity, simple color adjustment, and low synthesis cost, the possibility of development as light-emitters is very high. Since the perovskite materials can have a layered structure, in which a two-dimensional (2D) plane of an inorganic material is interposed between 2D planes of an organic material, and a dielectric constant difference between an inorganic material and an organic material is large (εorganic≈2.4, εinorganic≈6.1), electron-hole pairs (or excitons) are confined to the inorganic plane. Therefore, the perovskite materials having high color purity (full width at half maximum (FWHM)≈20 nm) are formed.
As a material having a perovskite (ABX3) structure in the related art, there is an inorganic metal oxide.
Such an inorganic metal oxide is generally oxide, and is a material in which cations of metals (such as an alkali metal, an alkaline earth metal, a transition metal, a lanthanide, and the like) such as Ti, Sr, Ca, Cs, Ba, Y, Gd, La, Fe, Mn, and the like having different sizes are located at A and B sites, anions of oxygen are located at an X site, and the cations of metals at the B site are combined with the anions of oxygen at the X site as a corner-sharing octahedron form of 6-fold coordination. As examples of the inorganic metal oxide, there are SrFeO3, LaMnO3, CaFeO3, and the like.
On the other hand, in the halide perovskite, cations of organic ammonium halide (RNH3) are located at an A site and halides (Cl, Br, and I) are located at an X site in an ABX3 structure to form an organic metal halide perovskite material, and thus a composition of the organic metal halide perovskite material is completely different from that of an inorganic metal oxide perovskite material.
Also, properties of the materials are based on the difference of such constituent materials. The inorganic metal oxide perovskite has representative properties such as superconductivity, ferroelectricity, colossal magnetoresistance, and the like. Therefore, the inorganic metal oxide perovskite is generally applied to sensors, fuel cells, memory devices, and the like, and research thereon has been conducted. For example, yttrium barium copper oxide has superconductivity or an insulation property according to oxygen content.
On the other hand, since a perovskite (or an organic metal halide perovskite) has a structure, in which an organic plane (or an alkali metal plane) and an inorganic plane can be alternately stacked, which is similar to a lamellar structure, excitons may be confined to the inorganic plane. Therefore, an ideal light-emitter which emits light having very high color purity may be formed by a crystalline structure itself rather than a size of the material in essence.
Even in the perovskites, since light emission occurs in organic ammonium cation when the organic ammonium contains a chromophore (mainly including a conjugated structure) having a smaller bandgap than that of a crystalline structure of a central metal and halogen (BX3), light having high color purity cannot be emitted, a FWHM of the light emission spectrum becomes wider than 50 nm, and thus the hybrid perovskite becomes unsuitable as a light emitting layer. Therefore, in this case, this type of organic-inorganic hybrid perovskites is not very suitable for a high color purity light-emitter emphasized in the present invention. Therefore, in order to make a high color purity light-emitter, it is important that organic ammonium as cation does not contain a chromophore and light emission occurs in an inorganic material lattice consisting of a central metal and a halogen element. That is, the present invention focuses on the development of a high color purity and high efficiency light-emitter in which light emission occurs in an inorganic material lattice. For example, in Korean Patent Application Publication No. 10-2001-0015084 (Published on Feb. 26, 2001), an electroluminescent element in which a dye-containing organic-inorganic hybrid material is formed in the form of a thin film rather than particles and is used as a light emitting layer is disclosed, but light emission originated from the emitting-dye itself not in a perovskite lattice structure.
However, since the perovskite has a small exciton binding energy, there is a fundamental problem in that, at low temperature, light can be emitted but, at room temperature, excitons cannot lead to light emission due to thermal ionization and delocalization of a charge carrier, are separated into free charges, and disappear. Also, when the free charges are recombined to form excitons, there is a problem in that the excitons are destroyed by a layer having a high conductivity around the excitons, so that light emission does not occur. Therefore, it is necessary to prevent quenching of the excitons in order to increase light luminescence efficiency and luminance of a perovskite based LED.